Several systems have attempted to deal with the issue of fires in air ducts. U.S. Pat. No. 6,105,678 discloses a heat responsive fire extinguishing assembly for a ventilating duct. The system includes a nozzle head that is mounted inside the duct. The nozzle head is in fluid communication with a reservoir containing the fire suppressant material via the valve. When the temperature inside the duct reaches a certain level, the valve is opened and the fire extinguishing material is dispensed through the nozzle in the duct. This automatic fire suppression system provides a nozzle head protruding into the duct undesirably blocking airflow through the duct.
U.S. Pat. No. 6,095,252 is a means for fighting fire in at least one cable or line run. A frangible tube is mounted inside a cable run. The tube is filled with fire extinguishing material under pressure. When the temperature inside the duct reaches a predetermined value, the tube ruptures releasing the fire extinguishing material into the cable. This reference depicts an automatic fire suppression system with a frangible tube adapted for suppressing fires in electrical cable runs.
U.S. Pat. No. 5,575,312 is a fire retarding heating and cooling duct. Heat meltable plastic is inserted into the duct at six foot intervals. When a fire occurs in the duct, the plastic melts almost immediately, sealing off the fire from the rest of the duct system. This type of duct stops the duct fire spreading to the rest of the ducting system.
U.S. Pat. No. 5,493,820 describes a fire preventing duct system. In this system a narrow tube containing cold water is inserted into the duct system. When a fire occurs, the heat is absorbed by the cold water, which impedes the progress of the fire. Once the temperature inside the duct reaches a predetermined value, the now hot water is flushed and replaced with cold water to absorb more heat, further impeding the progress of the fire. This system discloses a cooling system that removes heat from a duct system to delay the spread and growth of a potential duct fire.
U.S. Pat. No. 4,143,670 describes another system to impede the progress of a fire. The ducting fire protection system is created by placing a thermoplastic sleeve inside a duct. When a fire occurs, elevated temperatures inside the duct causes the sleeve to collapse, sealing off the fire from the rest of the duct system. This provides a device that seals off the duct fire from the rest of the ducting system.
U.S. Pat. No. 3,720,153 is another ducting fire protection system similar to the previous patent. In this system, a valve device is placed inside the duct. When a fire occurs, the elevated temperatures cause the value to close, stopping the fire from spreading to the rest of the duct system.